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Convection furnaces are used for the reflowing of solder in the surface mounting of electronic devices to circuit boards. In such furnaces, boards, having preformed solder and/or solder-paste patterns on the boards and devices, travel on a conveyor through a plurality of heating zones. Circulating gas in the furnace is heated by convection heaters above and below the conveyor and impinges on the boards and devices to heat the solder to the reflowing temperatures. The gas is typically circulated by a gas moving device, such as a fan. A cooling section downstream of the heating zones is generally used to cool the boards down from the reflowing temperature. Such furnaces are typically custom built to a customer""s specifications. Each of the heating and/or cooling zones is housed in an integral, insulated housing.
The heating zone of a furnace typically includes a hot air plenum comprising a plenum box that includes an orifice plate for diffusing the flow of hot gas over the product. A heating element and various other components, such as safety switches are housed within the plenum box. The hot air plenum is typically designed as a field-replaceable unit, so that the entire hot air plenum can be removed by unfastening a minimum number of fasteners and electrical connections. In this manner, if a component, such as a heater or blower motor, within the hot air plenum fails or the hot air plenum needs to be upgraded, the entire hot air plenum is removed and another hot air plenum installed in its place. This procedure minimizes process line downtime and the mean time to repair. Due to the large size and weight of the field replaceable unit, however, maneuvering the unit out of the furnace, particularly from the bottom part of the process chamber, can be difficult, especially for a single individual.
In most cases, the motor driving the blowers is located outside the furnace enclosure to protect the motor from heat and process gasses. In such cases, a shaft, often the motor shaft, penetrates the furnace enclosure to the blower, and a seal around the shaft keeps gas and heat within the enclosure and away from the motor. The shaft seal typically consists of mechanical contact seals on the shaft, or in some cases, one or more bearings surrounding the shaft independent of the motor, with purging chambers or other means to ensure separation of the enclosed and outside atmospheres.
Nitrogen, N2, is often used as an inerting gas to improve process yields in the reflow soldering of printed circuit boards. Upon start-up, oxygen, O2, in the process chamber is purged out until the N2 is at the desired level for the process. Besides the gas in the process tunnel, there is a large amount of O2 in the thermal insulation in the furnace housing. While it is easy to displace O2 in the tunnel with N2, O2 within the insulation is trapped behind metal insulation covers, which are necessary to prevent product contamination by insulation fibers. Unaided, the furnace takes a long time before the N2 level reaches the desired level, because the O2 diffuses out of the insulation at a slow but sustained rate. A common remedy is to purge the insulation from within by jets, tubes, manifolds, etc. However, this is wasteful in terms of gas consumption, costly and inefficient. Another possible solution is the use of a closed cell, non-permeable thermal insulation. This insulation is not as effective in insulating the furnace, however, because it is not able to fill all voids, corners, seams, etc. and block gas movement. This type of insulation may also give off undesirable gases or fumes.
The present invention provides a modular furnace fabricated from at least one module containing any desired number of process zones, such as heating and cooling zones. The modular construction allows a furnace design to be readily changed during manufacture or in the field. More particularly, each module has a frame, a removable cover mounted to the frame, exterior cover panels on the frame, and an insulated case fastened to the interior of the module.
In another aspect of the invention, field replaceable universal blower assemblies are associated with each process zone, resulting in a reduction in weight and volume of replaceable components. Also, the motor shaft in each blower motor is provided with a sealed bearing that allows the motor to be mounted to the process chamber without additional shaft seals but still providing a gas tight connection. Insulation within each module is held in place by gas permeable fabric covering large vent openings in cover panels to allow oxygen to diffuse out of the insulation rapidly upon start up. In a further aspect, a cooling unit may be provided to cool gas heated by the product and return the cooled gas to the cooling zone.